The first generation of optical fiber systems in the public telephone network used proprietary architectures, equipment line codes, multiplexing formats, and maintenance procedures. This diversity complicated the task of the Regional Bell Operating Companies and the interexchange carriers who needed to interface their equipment with these diverse systems.
To ease this task, Bellcore initiated an effort to establish a standard for connecting one optical fiber system to another. That standard is officially named the Synchronous Optical Network but it is more commonly called “SONET.” The international version of the standard is officially named the Synchronous Digital Hierarchy but it is more commonly called “SDH.”
Although differences exist between SONET and SDH, those differences are mostly in terminology. In virtually all practical aspects, the two standards are the same, and, therefore, virtually all equipment that complies with either the SONET standard or the SDH standard also complies with the other. For the purposes of this specification, the combined acronym/initialism “SONET/SDH” is defined as the Synchronous Optical Network or the Synchronous Digital Hierarchy or both.
FIG. 1 depicts a block diagram of a portion of a SONET/SDH network that receives a traffic signal at SONET/SDH switch 101 and transmits the signal from SONET/SDH switch 104. For example, the network might originally transmit the traffic signal from SONET/SDH switch 101 to SONET/SDH switch 104 through SONET/SDH switch 102. In the course of provisioning and deprovisioning the available resources, the need can arise whereby a traffic signal is carried from SONET/SDH switch 101 to SONET/SDH switch 104 through SONET/SDH switch 103, rather than through SONET/SDH switch 102. Analogously, the need can arise whereby a traffic signal is carried from SONET/SDH switch 101 to SONET/SDH switch 104 through SONET/SDH switch 102, rather than through SONET/SDH switch 103. These needs often occur when grooming is performed.
If there is a difference in the propagation delay through SONET/SDH switch 101 to SONET/SDH switch 104 through SONET/SDH switch 102, in contrast to the propagation delay through SONET/SDH switch 103, then the re-routing of the traffic signal through SONET/SDH switch 103 will cause the dropping, replacing, inserting, or repeating of at least one bit in the traffic signal. This is colloquially, although almost universally, called a “hit.” Hits are not advantageous and therefore hitless re-routing is preferred.
If the user of a SONET/SDH network experiences infrequent hits, the hits are most likely to be tolerated. In contrast, if the user of the SONET/SDH network experiences frequent hits, the hits are unlikely to be tolerated. This places a practical limit on the number of hits that the operator of a SONET/SDH network can inflict on its users. Furthermore, the inability to inflict an arbitrarily large number of hits on a user hinders the ability of the operator to groom the network, and the operator of the network must be capable of freely grooming the network so as to maintain the resource utilization of the network.
Therefore, the need exists for a technique for the re-routing of synchronous signals without causing a large number of hits.